The present invention relates generally to telescopes. More particularly, the present invention relates to a multi aperture telescope having a high fill factor.
Traditional telescopes often include a single collecting element (configured to collect electromagnetic radiation from a scene) and one or more secondary reflectors (configured to focus the collected electromagnetic radiation on an image plane). The first collecting element of a traditional telescope is typically disposed at the telescope's entrance pupil. An example of a typical refractor telescope 5 is shown in FIG. 1. Refractor telescope 5 includes a primary lens 10 disposed near an entrance pupil 12 and includes two smaller relay lenses 14 and 16. Primary lens 10 is configured to form an intermediate image of a scene at an intermediate image plane 18. Relay lens 14 is configured to collimate the collected electromagnetic radiation and transmit the electromagnetic radiation to relay lens 16. Relay lens 16 is configured to focus the collected electromagnetic radiation on an image plane 22. A real exit pupil 20 (an image of the entrance pupil) is positioned optically upstream of image plane 22. Refractor telescopes are typically relatively heavy due in part to the large amounts of glass used to form the refractive optical elements of such telescopes. Accordingly, refractor telescopes are generally not used for space-based applications, as the cost of launching such heavy telescopes is excessive. Moreover, refractive elements tend to become fogged in space deployment due to the cosmic particles that strike the refractive elements. Fogging tends to limit the amount of time refractive telescopes are useful in space applications.
As telescope systems are made larger, to achieve high resolution and to collect more light, a point is eventually reached where the size of the telescope elements, especially the primary mirror, exceeds the current state of the art in fabrication and mechanical support. One solution for overcoming this shortcoming, is to segment the primary mirrors into manageable pieces. For example, segmented elements forming a primary mirror can be made collectively lighter than a monolithic primary mirror. Another solution for overcoming the shortcomings of large traditional telescopes, is to form the telescope from a number of sub-aperture telescopes. Such telescope arrays are often referred to as multi-aperture telescope arrays.
FIG. 2 shows a cross-sectional view of an exemplary multi-aperture telescope 23. Multi-aperture telescope 23 includes first and second sub-aperture telescopes 25 and 29, respectively, which are disposed near an entrance pupil 26. First sub-aperture telescope 25 includes a primary element 24 and a secondary element 28, and second sub-aperture telescope 29 similarly includes a primary element 30 and a secondary element 32. Light collected by the sub-aperture telescopes is coherently combined at an image plane 36 (disposed behind an exit pupil 40) by a combiner lens 38. A set of flat fold mirrors 34 is configured to direct collected electromagnetic radiation to a combiner lens 38. Multi-aperture telescope 23 is an example of a sparse array telescope. That is, the fill factor of multi-aperture telescope 29 is less than about 50%. Fill factor may be defined as the percentage of electromagnetic radiation collected by the array of sub-aperture telescopes that enters an encircling entrance pupil of a multi-aperture telescope. Typical sparse array telescope designs, such as multi-aperture telescope 23, have relatively large spacings between sub-aperture telescopes to provide light beam clearance at the telescopes' combiner planes to prevent vignetting and optical element interference.
A need exists for a multi-aperture telescope having a high fill factor (e.g., larger than about 50%) so that relatively large amounts of electromagnetic radiation from scenes may be collected to form images of the scenes.